Cooling and height sensing system for a plasma arc cutting tool

ABSTRACT

A plasma arc cutting torch cools its nozzle with a water flow between an inner metallic nozzle member and an outer ceramic nozzle member. A set of auxiliary ports formed in the ceramic element each extend from an associated radial channel that directs a portion of the water to the plasma arc where it forms an annular &#34;jet&#34; that constricts the arc. The auxiliary ports are located and sized to provide an enhanced flow of water through the nozzle while maintaining an optimal flow rate through the radial channel. When a gas flow through the cooling passages is used to sense the height of the torch over a workpiece, the gas flow through the auxiliary ports clears residual water from the nozzle to avoid a false height sensing due to an emission of droplets of the water.

BACKGROUND OF THE INVENTION

This invention relates in general to plasma arc cutting torches and moreparticularly to an improved nozzle construction that provides enhancedcooling and a more reliable initial height sensing.

The present invention is an improvement over the cutting torches andheight sensing method described in U.S. Pat. Nos. 3,641,308 and4,203,022. The U.S. Pat. No. 3,641,308 patent describes a nozzle for aplasma arc cutting torch where a laminar jet of a cooling liquid,usually water, surrounds and constricts the plasma. The liquid jet iscreated by a pair of generally annular members which together form thenozzle of the torch. The inner member has a central passageway thatdirects an ionizable gas from an electrode to a workpiece located belowthe electrode. The outer member cooperates with the inner one to createan annular nozzle that emits the liquid jet. The effectiveness of thissystem is concentrating the arc depends, in part, on the water mass flowrate and flow velocity creating the constricting water jet.

The U.S. Pat. No. 4,203,022 patent describes a method and apparatus forsensing the height of the torch over a workpiece, particularly as thetorch is lowered toward the workpiece to begin cutting. Height sensingis important since the outer annular member of the torch nozzle istypically a ceramic material that can be damaged through contact withthe workpiece. The U.S. Pat. No. 4,203,022 patent teaches that a gasflow can be introduced through the gas system as the torch is lowered.The gas stream swirls and has a vortex pressure. When the torch isclosely spaced from the workpiece, the vortex "attaches" to theworkpiece resulting in an abrupt change in the vortex pressure. Thischange is sensed and gives the desired height information. One source ofunreliability with this system arises out of residual water which isoften present in the water ejection area of the nozzle, particularlywhen the nozzle is worn. Also, during the height sensing operation, if adroplet of residual water is ejected by the gas flow, the droplet cancause a pressure fluctuation which can be interpreted incorrectly as anindication that the torch is at the proper height over the workpiece.

More generally, the performance of a plasma arc cutting torch isdirectly related to the ability of the cutting system to cool the nozzleof the torch. The cooler the nozzle, the larger the current that theplasma can conduit. Also, a cool nozzle has an extended life since lessscale and deposits form on the nozzle. In a water cooled torch of thegeneral type shown in U.S. Pat. Nos. 3,641,308 and 4,203,022, heretoforeit has not been possible to simply increase cooling by increasing thewater flow because the laminar jet used to constrict the plasma issensitive to flow parameters.

It is therefore a principal object of this invention to provide a plasmaarc cutting torch that has its plasma arc concentrated by a jet of acooling liquid while at the same time offering markedly increasedcooling as compared to conventional torches of this type.

Another object is to provide a nozzle construction with an extendeduseful life.

A further object is to provide a nozzle construction which avoidsunreliability in the initial height sensing due to the presence ofresidual water in the nozzle.

Yet another object is to provide an improved plasma arc cutting torchwith the foregoing advantages which has a competitive cost ofmanufacture as compared to conventional torches of the same generaltype.

These and other objects and features will be more fully understood fromthe following detailed description which should be read in light of theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view in vertical section showing the lower portion of aplasma arc cutting torch constructed according to the present invention;and

FIG. 2 is a bottom plan view of the torch shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the lower portion of a plasma arc cutting torch 12 of thegeneral type described in U.S. Pat. Nos. 3,641,308 and 4,203,022 andmanufactured and sold by Hypertherm, Inc. of Hanover, New Hampshireunder the trade designation model PAC-500. A detailed description of thetorch and its support systems can be found in these patents and will notbe repeated here. This type of torch is conventionally operated togenerate a plasma arc in an ionizable gas such as nitrogen (N₂) with ahelical swirl being imparted to the gas within the torch to improve thecharacteristics of the cut. The arc extends generally from an electrode14 to a metal workpiece such as a steel plate (not shown). The torchalso uses a water injection system to cool its nozzle 16 and toconstrict the plasma arc as it is emitted from the torch.

The nozzle 16 has two principal components, an inner nozzle member 18and an outer member 20. The inner member 18 is typically made of copperand the outer element 20 is ceramic; both have a generally annularconfiguration. As shown, the ceramic member 20 generally surrounds thelateral and lower outer surfaces of the metallic member 18. The member18, in combination with the electrode 14 defines a flow path 22 for theplasma gas. The gas enters the path 22 at the point 22a with a helicalswirl imparted by a set of tangential ports in a ring (not shown)mounted just above the inner member. The plasma gas then flowsdownwardly through a central passageway 24 of the member 18 to theworkpiece.

The members 18 and 20 are in a generally abutting relationship with oneanother except for a generally radially arrayed channel 26 formed attheir interface and adapted to conduct a cooling fluid, typically water,from an annular region adjacent the upper lateral surfaces 18a and 20aof the members to a point 26a adjacent the plasma arc near its point ofexit from the torch. Cooled water flowing through the channel 26 coolsthe member 18. Radial jet of water ejected from the channel at the point26a creates a laminar jet of water which constricts and concentrates theplasma in the manner described in the aforementioned patents.

A principal feature of the present invention is a set of auxiliary ports28 formed in the ceramic, outer member 20. Each port 28 extends from thechannel 26 at a point 30 to the lower face 20b of the ceramic element.In the preferred form shown, there are eight auxiliary ports spacedgenerally equiangularly about the central passageway 24. Each port 28 isalso spaced radially from the passageway 24 a sufficient distance thatthe effluent flow of cooling water from the ports 28 does not interferewith the cut. To the same effect, the ports 28 are directed generallyvertically, parallel to the path of the arc from the electrode to theworkpiece. The location and concentration of the ports 28 can varyprovided that the effluent water does not interfere with the cut.

The ports 28 divert a portion of the cooling water from the channel 26before they transverse the final length 26c of the channel (extendingfrom the point 30 to the point 26a). As a result, there is an increasedmass flow rate and/or flow velocity over the preceding, upstreamportions 26d of the channel 26 as compared to conventional torches ofthis type where all of the nozzle cooling water is ejected into thelaminar jet that constrains the plasma. This increased flow provides agreater cooling of the nozzle which in turn allows the nozzle to beoperated at increased current levels or, for operation at conventionalcurrent levels, at a cooler temperature. With the present invention, ithas been found that it is possible to increase the maximum current byapproximately 25%. Cooler operating temperatures result in a longernozzle life since they are associated with less scale and deposits beingformed on the nozzle.

The dimensions of the ports 28 are selected in conjunction with those ofthe channel 26 so that the water flow through the channel portion 26c isat a sufficient rate and velocity to constrict the plasma arc as taughtin the aforementioned patents. For the Hypertherm model PAC-500 torchwith a 0.12 inch nozzle, the ports 28 should divert approximately 30% ofthe cooling water while 70% of the water flows through the channelportion 26c. A port diameter of approximately 0.032 inch has been foundto be satisfactory. For different torches, however, the dimensions andratios will, of course, vary. The percentage of water carried by theports 28 will usually lie in the range of 20% to 50%.

The nozzle construction of the present invention is also useful inconnection with the height sensing procedure which is the subject ofU.S. Pat. No. 4,203,022. In that procedure, the supply of water to thewater cooling system for the nozzle is shut off and a supply of gas isdirected through the system. The tangential ports create a swirlingmovement in the gas to generate a vortex. The vortex is weak until thetorch is close to the workpiece and "attaches" to it. This attachment isaccompanied by an abrupt drop in the vortex pressure which is sensed bya transducer.

One problem with this system has been the presence of residual water inthe channel 26 which can cause erratic pressure readings, commonlytermed "spitting", or can result in a false indication that the torch isproperly positioned due to the ejection of droplets of residual water.This latter situation is particularly troublesome when the nozzlebecomes worn at the site of ejection and the residual water film isunstable. With the present invention, during the initial height sensingprocess the gas flow drives residual water out of the nozzle through theports 28. The residual water therefore does not interfere with theheight sensing process. It should also be noted that the effluent gasflow from the ports 28 also does not interfere with the height sensing.

While the invention had been described with respect to its preferredembodiments, it will be understood that various modifications andvariations will occur to those skilled in the art from the foregoingdetailed description and the accompanying drawings. Such modificationsand variations are intended to fall within the scope of the appendedclaims.

What is claimed is:
 1. In a plasma arc cutting apparatus having a nozzlethrough which the plasma arc is ejected, said nozzle including (i) aninner member with a central passageway that directs said plasma arc froman electrode to a workpiece, (ii) an outer member which generallysurrounds said inner member, and (iii) a fluid passage formed generallybetween said inner and outer members which directs a flow of coolingliquid about said inner member, said fluid passage having a generallyradially directed channel portion adapted to establish a generallyannular column of said cooling liquid around said plasma, theimprovement comprisinga plurality of auxiliary ports formed in saidouter nozzle member, each of said ports (i) being in fluid communicationwith said fluid passage, (ii) extending to the outer surface of saidouter member in a direction that directs the cooling liquid carried bysaid ports away from the cutting area, and (iii) having a crosssectional dimension that provides an enhanced flow rate of the coolingfluid through said nozzle while providing a flow rate through saidchannels at a value conducive to maximize the efficiency of saidcutting.
 2. The improvement of claim 2 wherein said auxiliary ports eachextend generally in the direction of said plasma arc and are spacedradially from said annular column of cooling liquid.
 3. The improvementof claim 2 wherein said ports are each a passage having a generallycircular cross sections.
 4. The improvement of claim 2 wherein saidplurality of auxiliary ports is eight and said ports are generallyuniformly distributed about said central plasma passageway.
 5. Theimprovement of claim 1 wherein said cooling liquid flow through saidauxiliary ports is in the range of 20% to 50% of the total water flowthrough said nozzle.